[R-package] consolidate testing constants in helpers file

[jameslamb]

Working on #5988, I learned about the helpers.R mechanism in {testthat}.

That file is sourced before each test file is run (sort of like a conftest.py with pytest in Python). This PR proposes consolidating some constants that are duplicated across all the tests files into that file, to reduce duplication and encourage consistency across the tests.

For details on how {testthat} treats that file specially, see https://r-pkgs.org/testing-design.html#testthat-helper-files.

file that is always executed by load_all() and at the beginning of automated testing is a helper file, defined as any file below tests/testthat/ that begins with helper.

[github-actions]

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